Physeal injuries account for 30% of all pediatric fractures and can result in impaired bone growth. The physis (or, ?growth plate?) is a cartilage region at the end of children's long bones that is responsible for longitudinal bone growth. Once damaged, mesenchymal stem cells from the underlying subchondral bone migrate into the injured physis, undergo osteogenesis, and form unwanted bony tissue, referred to as a ?bony bar?. This can lead to angular deformities or completely halt longitudinal bone growth, which is devastating for children that are still growing. Current surgical treatments involve the removal of the bony bar. The site is often filled either with a soft fat graft or a hard, non-degradable plastic, both of which offer imperfect solutions leading to collapse of the resection site or the dislodgement of the biomaterial, respectively. Thus, the overall goal of this project is to develop an improved treatment option that utilizes 3D printing technology to engineer a biomimetic of growth plate cartilage containing mechanically-graded 3D stiff structures in-filled with a soft cartilage biomimetic hydrogel. Our hypothesis is that a 3D printed biomimetic of growth plate cartilage prevents collapse at the resection site through its structure and simultaneously recruits MSCs to direct them through zonally appropriate physiochemical cues to a chondrogenic, not osteogenic, lineage and prevents bony bar formation by replacing it with a cartilaginous repair tissue. Thus, long-term the 3D printed biomimetic will allow normal bone elongation after physeal injury. To test this hypothesis, we have developed two aims for the R21 phase and two aims for the R33 phase. In the R21 phase, we will (1) print a 3D construct that mimics the morphology and mechanical properties of growth plate cartilage (Aim 1) and (2) evaluate the ability of a 3D printed biomimetic of growth plate cartilage to prevent bony bar formation in a rabbit model of physeal injury (Aim 2). At the conclusion of the 2-year exploratory phase, we expect to have established a novel biomimetic of growth plate cartilage designed through 3D printing technology and confirmed that a 3D printed stiff structure mimicking that of the growth plate and infilled with a soft hydrogel prevents bony bar reformation. In the R33 phase, we will (1) assess cartilage formation in the implanted 3D printed biomimetic construct in a rabbit model of physeal injury through the recruitment of endogenous stem cells (Aim 3), and (2) evaluate the ability of a 3D printed biomimetic of growth plate cartilage to enable longitudinal bone growth in a rabbit model of physeal injury, which is followed for 1 year after implantation. At the conclusion of the 3-year R33 phase, we expect to have demonstrated that filling the site after bony bar resection with a 3D printed biomimetic of growth plate cartilage prevents bony bar reformation and supports cartilage formation that is eventually converted into new bone following growth to skeletal maturity. By providing a solution to restore normal bone growth, this 3D printed biomimetic of growth plate cartilage has the potential to be translated into the clinic to improve the quality of life of affected children.